A generic disc brake caliper is disclosed in German patent application No. 42 02 927. The prior art disc brake caliper includes a hydraulic actuating device, which is comprised of an open brake cylinder and a brake piston, which axially slidably arranged in the brake cylinder. The brake piston and the brake cylinder are sealed in relation to each other by means of a sealing ring, which is mounted in a circumferential annular groove in the inside of the brake cylinder and is in frictional abutment on an external peripheral surface of the brake piston for sealing purposes. The purpose of the sealing ring not only is to seal the interior of the brake cylinder. The sealing ring, due to its elastic properties, additionally assists in adjusting a clearance between the friction surfaces of the brake shoes and the brake disc after the brake application.
Upon brake application, hydraulic pressure is applied to the brake piston and causes it to move out of the brake cylinder in an axial direction. Upon axial displacement of the brake piston, the sealing ring, which bears against the external peripheral surface of the brake piston, is deformed elastically due to the friction (in first line: static friction) between the sealing ring and the brake piston. Upon termination of the brake application, i.e., when the hydraulic brake system is relieved from load, the elastic sealing ring will return to its original shape and shift the brake piston by a small amount into the brake cylinder. A brake shoe, which abuts the brake piston, follows the axial displacement of the brake piston, and its friction surface is lifted from the brake disc so that a clearance is maintained between the brake shoe and the brake disc.
The amount of the adjusted clearance generally depends on the characteristics of the sealing ring and the annular groove accommodating the sealing ring, especially the cross-sectional shape of the annular groove. Another undesirable relationship is associated with the hydraulic pressure, which prevails in the brake cylinder during brake application: The higher the hydraulic pressure, the more the sealing ring deforms. Attempts have generally been made to maintain an unchanging constant clearance after each brake application irrespective of the hydraulic pressure that occurs. To achieve this objective, a plurality of various cross-sectional shapes for the annular groove have been proposed so far. However, none of the cross-sectional shapes permitted achieving a fixed clearance irrespective of the hydraulic pressure.
For example, in the disc brake caliper disclosed in German patent application No. 42 02 927, a transition area, which is defined by adjacent circumferential conical surfaces or torus surfaces, is provided between the annular groove wall and the cylinder surface. The sealing ring is urged, at least in part, into this transition area during brake application. However, this embodiment does not either ensure a pressure-independent clearance. Further, damage to the sealing ring may be caused, especially when high hydraulic pressures are applied, because the sealing ring will yield far into the acutely tapering transition area in this case. This will greatly reduce the useful life of a sealing ring of this type.
An object of the present invention is to improve a generic disc brake caliper with respect to its clearance. The special objective is to achieve a fixed clearance irrespective of the hydraulic pressure applied.
This object is achieved by the present invention by fashioning a circumferential recess in the transition area between the brake cylinder surface and the groove wall of the annular groove, which is oriented substantially vertically to the brake piston axis. The recess is defined in its cross-section by a contour line, which starts from the groove wall in a convex curvature and passes over into a concave curvature. Depending on the hydraulic pressure applied, a corresponding elastic deformation of the sealing ring into the circumferential recess is permitted. The circumferential recess is in first line arranged at that groove wall of the annular groove, which is closest to the open end of the brake cylinder.
To minimize damages to the material of the elastic sealing ring, the sectionwise different radii of curvature of the contour line of the cross-section of the recess will continuously pass into one another in a preferred embodiment of the disc brake caliper. The fabrication of the recess may be simplified by defining the recess at least in sections by circumferential conical partial surfaces.
In a preferred aspect of the cross-section of the recess, the contour line and the brake cylinder surface form a preferably right angle. This reduces symptoms of wear on the sealing ring, especially at high hydraulic pressures.
Another preferred aspect of the cross-section of the recess is obtained by providing a radial elevation in the contour line so that the free space available for the elastic deformation of the sealing ring is contracted. This avoids an excessive deformation of the sealing ring into the recess at medium hydraulic pressures.